Production of phosphoric acid by what is commonly known as the “wet process” involves the reaction of finely ground phosphate rock with sulfuric acid. As a result of the various reactions, a slurry is produced containing phosphoric acid, calcium sulfate and various impurities derived from the phosphate rock. The slurry is normally filtered to separate the phosphoric acid product from the by-product calcium sulfate. The phosphoric acid obtained is then used in the production of various phosphate fertilizers, such as ammoniacal fertilizers, via neutralization of the acid with ammonia.
Several varieties of wet process production of phosphoric acid are utilized around the world. The most commonly employed is the Di-Hydrate process, in which a specific crystalline form of calcium sulfate is produced by reaction of the calcium present in the raw phosphate ore and sulfuric acid used to acidulate the ore. Approximately 5 tons of calcium sulfate, or gypsum, are formed per ton of phosphate (P2O5) produced.
Water is normally used to wash the calcium sulfate filter cake and thereby increase the recovery of the phosphoric acid product. Most of this wash water is fed back into the phosphoric acid production process as make-up water. However, a portion of this water, together with some residual phosphoric acid, remains trapped in the calcium sulfate filter cake and is discharged from the filter with the filter cake. The trapped water contains several percent of phosphoric acid and small amounts of other impurities that were present in the phosphoric acid product. Additional water is normally used to help discharge the calcium sulfate filter cake off of the filter. Large volumes of water are then used to transport the calcium sulfate filter cake by pumping as a slurry to a storage or disposal area.
At the storage or disposal area the calcium sulfate will settle and the excess transportation water will be liberated. This liberated water will normally be collected in a system of channels and ponds and recycled to the phosphoric acid production plant for reuse. The pond water is used for washing the calcium sulfate filter cake and for cooling and scrubbing process vapors. The water can also be used in grinding the rock to produce a slurry and other purposes connected with the plant that do not require fresh water. These channels and ponds also serve as a collection site for other water that is used in and around the phosphoric acid plant, such as for cleaning or washing, fresh water fume scrubbers, and as a collection site for phosphoric acid spills or leaks within the plant. Also, since these channels and ponds are located outside, they collect rain water.
The water contained within these channels and ponds contains small amounts of phosphoric acid and other impurities normally present in the phosphoric acid. Consequently, it is considered contaminated. Most particularly, the recycled or process water contains about 0.25% to 3% phosphoric acid, similar amounts of fluoride species, several hundred milligrams per liter of soluble ammonia, and trace amounts of many heavy and toxic metals. Thus, it is really not water in the traditional sense, but rather a weak but very acidic solution. Before the water can be released to the environment, it must be treated or purified to remove the phosphoric acid and other impurities. In some cases, in an efficiently operated phosphoric acid plant, in the absence of severe weather conditions, a balance will exist between water input to the pond system and water evaporation such that virtually all of this contaminated water can be recycled and used within the plant. In this case, treatment and discharge of the contaminated water, commonly known as pond water, is not necessary.
However, there are circumstances under which treatment and discharge of the contaminated pond water is necessary. One such circumstance could be an extended period of abnormally heavy rainfall. When the climatic cycle is such that rainfall and evaporation do not match, and one significantly outweighs the other, such as in the case of significant storm events or a drought, the process water balance is severely impacted. Of greatest consequence is the situation where several storms cause the collection and mixing of large quantities of storm water with the process water and raising of the water inventories to unsafe levels. Environmental damage can occur if the process water is abruptly discharged into local streams and rivers. Thus, it is necessary to routinely reduce excessive contaminated water (pond water) inventories to prevent its accidental discharge and the attendant environmental impact. Another circumstance indicating treatment and discharge would be when the phosphoric acid plant has ceased operation either for an extended period of time or permanently.
Many factors influence the specific components and their concentrations in this contaminated pond water. While it cannot be said that there is any typical composition for pond water some of the major chemical components that could be found in pond water, and an example of their range of concentrations, are as follows:
CHEMICAL COMPONENTRANGE OF CONCENTRATIONP1000-12,000ppmSO44300-9600ppmF50-15,000ppmSi30-4100ppm(ammoniacal) N40-1500ppmNa1200-2500ppmMg160-510ppmCa450-3500ppmK80-370ppmFe5-350ppmAl10-430ppmCl10-300ppm
Normally the major acidic components of pond water are phosphoric acid and sulfuric acid, with lesser amounts of hydrofluorosilicic acid (H2SiF6), and hydrofluoric acid (HF). In an operating phosphoric acid complex, the pond water is normally saturated or supersaturated with respect to many of the ions contained within it. The only exception would be immediately after a period of extremely heavy rainfall. Also, since the pond water is used for cooling it undergoes continuous thermal cycling (heating and cooling). This thermally cycled water, along with the addition of some waste fresh water to the pond water, is why the pond water can function as an effective scrubbing fluid for some process gasses. One method of treating or purifying this pond water that has, until now, been the industry standard is well known in the art is double liming. This method consists of adding a calcium compound (such as CaCO3, Ca(OH)2 or CaO) to the pond water, in two stages, such that the fluoride, phosphate and other impurities form solid precipitates that settle and are separated from the thus purified water. This method is described in Francis T. Nielsson, ed., Manual of Fertilizer Processing, Marcel Dekker, Inc. (1987), pp. 480 to 482; G. A. Mooney, et al., Removal of Fluoride and Phosphorus from Phosphoric Acid Wastes with Two Stage Line Treatment, Proceedings of the 33rd Industrial Waste Conference, Purdue Univ. (1978); G. A. Mooney et al., Laboratory and Pilot Treatment of Phosphoric Acid Wastewaters, presented at the Joint Meeting of Central Florida and Peninsular, Florida A.I.Ch.E. (1977); and U.S. Pat. Nos. 5,112,499; 4,698,163; 4,320,012; 4,171,342; 3,725,265 and 3,551,332.
During the first stage lime treatment, lime is added to a pH of about 4-5, resulting in the precipitation of fluoride as CaF2 and/or CaSiF6. During this stage it is also thought that the hydrofluorosilicic acid present dissociates to HF and SiF4, with the SiF4 hydrolyzing to HF and SiO2. Some phosphate is also precipitated at this stage as Ca3(PO4)2 (calcium phosphate), as well as some CaSO4, (calcium sulphate). The sludge (CaF2, CaSiF6, Ca3(PO4)2 and other compounds) produced at this stage is a granular, crystalline material that settles fairly rapidly and can be de-watered to about 30% solids in a gravity thickener. The sludge can be sent to the plant gypsum stack or recycled to the phosphoric acid plant for recovery of the phosphate.
In the second stage, additional lime is added to the clarified liquid from the first stage to a pH of about 8-10. In this stage the remaining phosphates and fluorides are precipitated along with sulfate and many of the metals. The sludge in this stage has poor settling and thickening properties, due to the hydroxide nature of many of the compounds, and rarely achieves more than 5%-7% solids by weight. The sludge from this stage is normally deposited in large lagoons to allow for additional de-watering. In the treatment of process water containing unacceptable levels of soluble ammonia, the second stage system can be operated at a higher pH, from pH 10 to 12, to increase the un-ionized ammonia concentration, raising its volatility. The increased vapor pressure of the ammonia in solution facilitates its removal through air stripping by the addition of spray devices located in or floating on the sedimentation lagoon. Even without the addition of a spray system, operation of the treatment lagoon at a pH of 9 to 10 or greater results in the removal of ammonia through volatilization due to the large surface area available for such activity.2NH4++OH=H2O+NH3 (Unionized and volatile)
The quantity of clear water that can be obtained from a double liming process is about 50%-70% of the feed volume. As an alternative, liming procedures can be carried out in a single stage.
However, there are several problems associated with this method. One problem is the large volume of sludge produced. Sludge (i.e., a mixture of the precipitated impurities, un-reacted calcium compound and water) is produced in both the first and second stages of this process. These sludge materials, some of which de-water slowly, are normally deposited in settling ponds that require large land areas. Another significant problem with this treatment process is that very large volumes of lime are required to neutralize the acidic pond waters, some of which can have a pH as low as one.
Another general method of water purification is reverse osmosis. This process is based on the application of external pressure on an aqueous salt solution in contact with a semi permeable membrane, such that the applied pressure exceeds the osmotic pressure of the water component of the solution in contact with the membrane. Thus, some of the water is forced through the membrane in the reverse direction, while the other components in the solution (i.e., soluble salts) do not pass through the membrane. This results in a stream of purified water, known as permeate, and a stream of increased salt content, known as the reject or concentrate. Reverse osmosis is well known in the art and is described in Douglas M. Ruthven, ed., Encyclopedia of Separation Technology, Volume 2, pp. 1398-1430, John Wiley & Sons, Inc. (1997); S. Sourirajan and T. Matssuura, Reverse Osmosis/Ultrafiltration Principles, National Research Council of Canada, Ottawa, Canada (1985); B. Parekh, ed., Reverse Osmosis Technology, Marcel Dekker, Inc., New York (1988); R. Rautenbach and R. Albrecht, Membrane Processes, John Wiley & Sons, Inc., New York (1989) and other publications. Reverse osmosis is also described in a variety of U.S. patents, for example, U.S. Pat. Nos. 4,110,219; 4,574,049; 4,876,002; 5,006,234; 5,133,958 and 6,190,558.
Several attempts have been made to use reverse osmosis for the purification of contaminated phosphoric acid plant pond water. However, these attempts have generally failed due to the fact that the pond water is a saturated solution. Thus, as soon as any water is removed from the pond water the solution becomes supersaturated and salts precipitate and quickly clog the membranes used in reverse osmosis and prevent additional pure water from flowing through them. Also, even when the pond water was from a closed plant where rainwater had diluted it below saturation levels, an R.O. system can only remove a relatively small amount of purified water from the pond water before it again becomes a saturated solution. The use of anti-scalants can mitigate to some degree the scaling tendencies, but yields are often no more than 25% and frequent cleaning of the membranes is still required.
However, in an idle plant or at a phosphoric manufacturing facility being permanently closed, and where there is no adjacent operational facility, the concentrated reject stream has no home and no commercial value due to its dilute concentration. Typically it is returned to the pond system and must be re-processed by lime neutralization. Thus the overall consumption of lime to neutralize the water is unchanged whether the pond water is treated conventionally by double liming or is separated at an intermediate point and the reject stream further neutralized by lime. Thus there is no reduction in lime consumption by applying reverse osmosis technology in a non-operating facility.
A long-felt but unfulfilled need addressed by the present invention is to provide a process for the treatment of phosphate production waste acidic process water that will require less expensive capital equipment being required for the various separation stages and provide a more concentrated solids stream.